Heretofore, electronic parts such as diodes, transistors and integrated circuits have been mainly encapsulated with epoxy resin compositions. In order to secure safety, it is required as obligation by UL standards to impart flame retardancy to the epoxy resin composition which can be used as this semiconductor encapsulating resin. Accordingly, the above epoxy resin composition is usually blended with a halogen-based flame retardant material as a flame retardant material and diantimony trioxide as a flame retardant auxiliary. However, with the increase of consciousness regarding environmental problems in recent years, a high safety is required for the flame retardant material and the flame retardant auxiliary which are used in the encapsulating resins of various kinds of semiconductor devices.
The halogen-based flame retardant material generates a harmful halogen-based gas and the like during combustion, and there is anxiety that diantimony trioxide which is the flame retardant auxiliary has chronic toxicity. Accordingly, environmental and sanitary problems of the flame retardant material and the flame retardant auxiliary have been indicated, and so it is now considered that the conventional semiconductor encapsulating resins are insufficient for the safety. In addition, a halogen and antimony derived from the above flame retardant material and flame retardant auxiliary accelerate the corrosion of wires in the semiconductor device at a high temperature, particularly the corrosion of an interface between a gold (Au) wire and an aluminum (Al) pad (an interface between different kinds of metals) and the like, so that contact resistance between the Au wire and the Al pad increases. In consequence, a phenomenon such as wire breakage occurs, which causes the reliability of the semiconductor device, particularly wire corrosion resistance to lower at the high temperature. Therefore, it has been required to develop the epoxy resin composition for semiconductor encapsulate which is excellent the flame retardancy and reliability without using the halogen-based flame retardant material and diantimony trioxide.
For the above requirement, phosphorus-based flame retardant materials such as red phosphorus and phosphates, which have now been begun to be used in some fields, are useful to make the epoxy resin composition flame retardant, but each of these compounds tends to react with a trace amount of water to produce phosphine and corrosive phosphoric acids, and hence, humidity resistance is poor. Therefore, these phosphorus-based flame retardant materials are not suitable for the encapsulate of electronic parts in which the humidity resistance is extremely strictly required.
Furthermore, it has also been investigated to use metallic hydroxides such as aluminum hydroxide and magnesium hydroxide as well as boron-based compounds as the flame retardant materials, but unless each of these metallic hydroxides and boron-based compounds is used in large quantity to the epoxy resin composition, a sufficient flame retardant effect cannot be exerted. However, if a large amount of such a flame retardant material is added, the moldability of the epoxy resin composition deteriorates inconveniently.
In place of the use of the above halogen-based or phosphorus-based flame retardant material, there have been suggested an epoxy resin composition for encapsulating a semiconductor and a semiconductor device in which an inorganic filler is highly filled into the epoxy resin composition in a high ratio of, e.g., 87 to 95 wt % to improve the flame retardancy (Japanese Patent Application Laid-Open No. 301984/1996), and an epoxy resin composition and a semiconductor encapsulating device in which the inorganic filler is highly filled into the epoxy resin composition in a ratio of 83 vol % (91 wt % in terms of spherical silica powder) or more to improve burning resistance (Japanese Patent Application Laid-Open No. 208808/1997). In each of these epoxy resin compositions, however, the inorganic filler is highly filled, and hence, the moldability of the epoxy resin composition which is used to encapsulate the semiconductor device is inconveniently poor.
On the contrary, with regard to a technique of making the resin itself flame retardant without adding any flame retardant material, it has been heretofore mainly investigated to improve the heat resistance (thermal decomposition resistance) of a resin structure constituting a cured article of the epoxy resin. Such a technique can increase the density of a crosslinked structure formed by a curing reaction between resin components such as an epoxy resin and a phenolic resin in the cured article of the epoxy resin, whereby the molecular vibration of these resin components which occurs during heating and at ignition is limited. In consequence, the thermal decomposition of these resin components can be inhibited, and the amount of a decomposition gas including combustible components generated in such a case can be reduced, so that the combustion of the resin components is minimized to improve the flame retardancy of the cured article of the epoxy resin. However, as a result of the investigation of this technique, the sufficient flame retardancy could not be obtained.